Re: [RFC PATCHC 2/3] idle: store the idle state the cpu is

[Daniel Lezcano]

On 04/15/2014 04:33 PM, Peter Zijlstra wrote:
> On Tue, Apr 15, 2014 at 04:17:36PM +0200, Daniel Lezcano wrote:
> > On 04/15/2014 02:44 PM, Peter Zijlstra wrote:
> > > On Tue, Apr 15, 2014 at 02:43:30PM +0200, Peter Zijlstra wrote:
> > > > On Fri, Mar 28, 2014 at 01:29:55PM +0100, Daniel Lezcano wrote:
> > > > > @@ -143,6 +145,10 @@ static int cpuidle_idle_call(void)
> > > > >   			if (!ret) {
> > > > >   				trace_cpu_idle_rcuidle(next_state, dev->cpu);
> > > > >
> > > > > +				*power = &drv->states[next_state].power;
> > > > > +
> > > > > +				wmb();
> > > > > +
> > > >
> > > > I very much suspect you meant: smp_wmb(), as I don't see the hardware
> > > > reading that pointer, therefore UP wouldn't care. Also, any and all
> > > > barriers should come with a comment that describes the data ordering and
> > > > points to the matchin barriers.
> > >
> > > Furthermore, this patch fails to describe the life-time rules of the
> > > object placed there. Can the objected pointed to ever disappear?
> >
> > Hi Peter,
> >
> > thanks for reviewing the patches.
> >
> > There are a couple of situations where a cpuidle state can disappear:
> >
> > 1. For x86/acpi with dynamic c-states, when a laptop switches from battery
> > to AC that could result on removing the deeper idle state. The acpi driver
> > triggers:
> >
> > 'acpi_processor_cst_has_changed' which will call 'cpuidle_pause_and_lock'.
> > This one will call 'cpuidle_uninstall_idle_handler' which in turn calls
> > 'kick_all_cpus_sync'.
> >
> > All cpus will exit their idle state and the pointed object will be set to
> > NULL again.
> >
> > 2. The cpuidle driver is unloaded. Logically that could happen but not in
> > practice because the drivers are always compiled in and 95% of the drivers
> > are not coded to unregister the driver. Anyway ...
> >
> > The unloading code must call 'cpuidle_unregister_device', that calls
> > 'cpuidle_pause_and_lock' leading to 'kick_all_cpus_sync'.
> >
> > IIUC, the race can happen if we take the pointer and then one of these two
> > situation occurs at the same moment.
> >
> > As the function 'find_idlest_cpu' is inside a rcu_read_lock may be a
> > rcu_barrier in 'cpuidle_pause_and_lock' or 'cpuidle_uninstall_idle_handler'
> > should suffice, no ?
>
> Indeed. But be sure to document this.

Yes, sure. Thanks for pointing this.

  -- Daniel


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